Engine exhaust catalysts must reach a light-off temperature before they are activated and are generally inefficient at ambient air temperatures. Exhaust emissions generated below light-off temperatures of the catalyst may contribute a large percentage of the total exhaust emissions. Thus engine control systems may maintain exhaust catalysts at or above the light-off temperature.
One example approach for maintaining exhaust catalyst temperatures in a variable displacement engine is described by Denari et al. in U.S. Pat. No. 6,164,065. Therein, a plurality of control valves is used to direct exhaust flow from alternate cylinder banks through various paths in order to maintain the temperature of a downstream NOx trap catalyst. The frequency of alternating between the banks is adjusted so as to periodically reverse flow of exhaust gas through the NOx trap and control the catalyst temperature, while a hydrocarbon (HC) trap is used to ensure that exhaust HCs are not released before the catalyst is active.
However, the inventors herein have identified potential issues with such an approach. As one example, there may be conditions during which the same bank of the variable displacement engine is consecutively disabled. During such conditions, in the absence of exhaust gas flow reversals, the NOx trap catalyst temperature may not be maintained, thereby degrading catalyst activity. As another example, the system of Denari uses multiple valves requiring precise control and coordination. As such, this increases component cost and complexity.
Thus, in one example, some of the above issues may be at least partly addressed by a method of operating an engine including a first and second cylinder group. In one embodiment, the method comprises, selecting either the first or second cylinder group for deactivation, and closing a valve coupled between the selected cylinder group and a downstream catalyst based on the selection. In this way, by limiting air flow through the catalyst of the deactivated cylinder group, catalyst temperatures may be maintained within a desired range.
In one example, an engine with a first group and a second group of cylinders may be configured with a first valve coupled between the first cylinder group and a first downstream exhaust catalyst, and a second valve coupled between the second cylinder group and a second downstream exhaust catalyst. An engine control system may select a cylinder group for deactivation based on the regeneration state and temperature of the catalysts, and further based on a deactivation order. When the first cylinder group is deactivated, the first valve may be closed to limit air flow through the first catalyst. Additionally, the second valve may be adjusted to vary an amount of exhaust gas recirculated through the second cylinder group. As such, this allows engine EGR and VDE operations to be coordinated. Likewise, when the second cylinder group is deactivated, the second valve may be closed to limit air flow through the second catalyst, while the first valve is adjusted to vary an amount of exhaust gas recirculated through the first cylinder group.
In this way, by limiting air flow through a catalyst during cylinder deactivation conditions, catalyst temperatures may be maintained, thereby improving catalyst efficiency when the cylinders are reactivated. As such, this may improve exhaust emissions and engine fuel economy.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.